Thyristors which can be turned off via the gate, hereinafter termed GTO's for short, are at present able to block voltages of up to 4,500 V and turn off currents of up to 2,500 A. Such semi-conductor structures are in general composed of a large number of integrated individual thyristors with four-layer pnpn structures connected in parallel. The integrated individual thyristors have via common anode layers, a common n-type base layer, a common p-type base layer which is in contact with a gate, but via separate cathode layers. The cathode layers are highly doped and act as n+ emitters.
Such a GTO is described for example in EP-A2-0,066,850. The integrated individual thyristors are disposed on several concentric circles on a circular semiconductor substrate.
In order that high currents can be reliably switched, certain dimensioning criteria, principally in the region of the cathode and of the p-type base layer, have to be precisely adhered to. The switchable current depends, in particular, to a considerable extent on the transverse conductivity .sigma..sub.p of the p-type base layer.
In the case of a singly diffused p-type base layer, the conductivity can be improved only by increasing the number of mobile charge carriers. This can be achieved only by a greater layer thickness or by a higher doping density at the pn junction between cathode layer and p-type base layer. However, both these modifications have disadvantageous consequences. A greater thickness of the p-type base layer reduces the gain of the subsidiary npn transistor formed by n-type base layer, p-type base layer and cathode layer, as a result of which the GTO becomes less susceptible to triggering and slower to turn on, and a higher doping density in the region of the pn junction with the cathode layer lowers the breakdown voltage of said pn junction so that the risk of an avalanche breakdown on switching off the GTO becomes greater.
From the article entitled "Gate Turn-off Thyristors", Semiconductor Devices for Power Conditioning, Plenum Press, New York 1982, by M. Kurata et al, it is known that the switchable anode current increases in proportion to the product of the breakdown voltage V.sub.BJ1 of the pn junction J.sub.1 between cathode layer and p-type base layer multiplied by the layer conductance op of the p-type base layer. Accordingly, a higher breakdown voltage V.sub.BJ1 likewise results in higher switchable anode currents.
The breakdown voltage of a planar pn junction is known to depend substantially on the doping gradient at that point: the greater the doping gradient, the lower is the breakdown voltage. But in the case of a diffusion, however, maximum doping density, penetration depth and doping gradient are dependent on each other. It is therefore not possible to produce in this manner a cathode layer with high doping density, i.e. good emitter action, low penetration depth, which, because of the large part of the p-type base layer available, yields a good transverse conductivity .sigma..sub.p, and with a low doping gradient at the pn junction, i.e. a high breakdown voltage.